This is as far as I know the 'official' list with legit achievements.

Not sure if there is a bug with bnet, but apparently there are still tons of unmerged accounts around that are being sold by the dodgy spammers we all see every day in trade chat. Those are stolen (hacked) accounts from people who never returned to the game and from before the current bnet, and you can merge those with your bnet account (which is fraud and bannable obviously). Those accounts might contain some rare/unobtainable achievements/mounts/feat of strengths/titles and obviously if they become part of your bnet account you acquire those as well. Don't think they last long though as this should be pretty obvious to detect.